1. Field of the Invention
This invention relates to automated guided vehicles and, more particularly, to automated guided vehicles that comprise sensors or antenna coils for guiding the vehicles along a wire imbedded in a floor.
2. Description of Related Art
Automated guided vehicles which utilize guide wire navigation are known in the art. One example of such a vehicle is an automated lift truck, which typically operates along narrow aisles between, e.g., shelves in a warehouse. These vehicles comprise sensors or antenna coils which respond to magnetic fields generated by guide wires imbedded in the floor along a predetermined path. The voltages induced in the coils are representative of the lateral displacement error, or lateral distance from the wire to the centerline of the coils, and the heading angle error, or the angle between the wire and the centerline of the coils. The induced voltages are used to generate a steering correction error signal, which in turn is used to control a motor or similar mechanism which can turn the steerable wheel to maintain the coils, and therefore the vehicle, aligned with the wire as the vehicle is moving. Thus, the coils provide a means by which the vehicle can track along the wire and thereby stay within a predefined path. Typically, an operator is required to align the vehicle over the wire, although some automated guided vehicles are capable of acquiring the wire, or aligning the coils with the wire, to some degree once the vehicle is placed over the wire.
Examples of such automated guided vehicles are disclosed in U.S. Pat. No. 4,307,329, issued to Taylor, and U.S. Pat. No. 4,310,789, issued to Mank et al. These patents recognize that, to achieve close tracking at high speeds, the coils should provide a steering correction signal that is much more responsive to the heading angle error than to the lateral displacement error. This is due to the fact that lateral displacement error increases relative to the time integral of the vehicle velocity multiplied by the heading error. The patents also recognize that disruptions in the magnetic field created by floor anomalies, such as reinforcing bars or expansion joints, cause changes in the voltages induced in the coils that can cause tracking errors. Taylor employs two coil pairs and associated circuitry to generate a steering correction error signal that corresponds to a desired ratio of heading error to displacement error that will provide relatively good tracking along a straight wire. The coils in each pair are spaced 9.5 inches apart and the pairs are spaced 16 inches apart. This spacing is selected as a compromise that would best allow the vehicle to acquire the wire and track along a straight wire. However, Taylor's vehicle cannot track along a curve in the wire and may have problems acquiring the wire if the initial angle between the centerline of the coils and the wire is too great. In addition, the coils are so far apart that floor anomalies may cause tracking errors. While Mank et al. teaches using a third set of coils to determine the radius of a curve in the wire and thereby track along the curve, their vehicle may still be subject to tracking errors caused by floor anomalies and may have problems acquiring the wire when the initial angle is too great.